1. Field of the Invention
The invention relates to a resonance power circuit in which a voltage pulse is generated by the resonance operation on the primary side of a transformer and the voltage pulse is boosted by the transformer to be output from the secondary side thereof.
2. Discussion of the Related Art
A resonance power circuit used in a television receiver or a display device incorporates a stabilizing circuit for stabilizing a high output voltage (e.g. 30 kV) which is to be supplied from a flyback transformer to a cathode ray tube (CRT).
Stabilizing circuits for this purpose are classified into the type in which the voltage of a driving power source on the primary side of a flyback transformer is controlled, and the secondary control type in which a voltage for compensating a drop of a high output voltage is added to the secondary side of a flyback transformer.
However, a stabilizing circuit of the former type has a problem in that the response of the control is very poor, and that of the latter type has problems in that the correction range is narrow, and that a complex circuit is required.
In order to solve these problems as far as possible, recently, there has been proposed a system in which the primary current of a flyback transformer is directly controlled, in Japanese Patent Unexamined Publication (Kokai) No. Hei 2-222,374. According to the proposed system, the control range of a high output voltage can be widened. On the other hand, the proposed system requires a plurality of switching devices for controlling the primary current, and therefore has problems in that the number of parts is increased, and the circuit configuration is complicated. Moreover, the proposed system has further problems in that a circulating current flows uselessly to increase the power loss, and the switching operation of the switching devices during a scanning period causes noise.
The inventors have studied to a basic circuit of a resonance power circuit such as shown in FIG. 1, which is of the type wherein the primary current is directly controlled, and have vigorously labored to improve its circuit characteristics.
In FIG. 1, the positive terminal of a driving power source 3 is connected to one terminal of a primary coil 2 of a flyback transformer 1, and the negative terminal of the driving power source 3 is grounded. A transistor 4 which functions as a switching device is connected in series to the other terminal of the primary coil 2. A damper diode 5 and a resonance capacitor 6 are connected in parallel to the transistor 4, respectively.
The high voltage terminal of a secondary coil 7 of the flyback transformer 1 is connected to the anode of a CRT 10 through a high voltage rectifier diode 8.
In the circuit of this type, during the ON period (transistor period) of the transistor 4, a current shown in FIG. 2(b) flows from the driving power source 3 toward the transistor 4 through the primary coil 2, whereby an electromagnetic energy is accumulated in the primary coil 2. When the transistor 4 is then turned OFF, series resonance of the primary coil 2 and the resonance capacitor 6 starts so that the electromagnetic energy accumulated in the primary coil 2 is converted into an electrostatic energy of the resonance capacitor 6, thereby generating a flyback pulse (voltage pulse) as shown in FIG. 2(a). The flyback pulse has a peak value when all the electromagnetic energy accumulated in the primary coil 2 has been converted into the electrostatic energy of the resonance capacitor 6.
After the flyback pulse reaches the peak, the electrostatic energy of the resonance capacitor 6 is inversely converted into the electromagnetic energy of the primary coil 2, resulting in that the level of the flyback pulse gradually decreases. When the level of the pulse reaches zero, that is, the voltage of the point A of the circuit of FIG. 1 becomes zero, the damper diode 5 is rendered conductive so that a reverse current flows from the ground toward the primary coil 2. As a result, when the voltage of the point A returns to the power source voltage of the driving power source 3, the damper diode 5 becomes nonconductive. When the transistor 4 is again turned ON, the circuit returns to the initial operation state. The circuit continues to operate while repeating these operations. The flyback pulse generated in the side of the primary coil 2 is boosted by the flyback transformer 1 and then supplied to the anode of the CRT 10 through the high voltage rectifier diode 8.
The pulse voltage V.sub.c of the flyback pulse generated on the primary side of the flyback transformer 1 is indicated by an expression of V.sub.c =E.sub.B +r.sub.a sin(.omega.t-.phi..sub.a) where t is the time, E.sub.B is the power source voltage of the driving power source 3, and r.sub.a, .omega., and .phi..sub.a are indicated respectively as follows: EQU r.sub.a ={E.sub.B.sup.2 +(I.sub.0 /C.omega.).sup.2 }.sup.-1/2 EQU .omega.=(LC).sup.-1/2 EQU .omega..sub.a =tan.sup.-1 (E.sub.B C.omega./I.sub.0)
In the above expressions, I.sub.0 is the current flowing through the primary coil 2, C is the capacitance of the resonance capacitor 6, and L is the inductance of the primary coil 2.
As seen from the expression of V.sub.c, the level of the flyback pulse has a peak when .omega.t-.phi..sub.a =.pi./2. At this time, the peak voltage is V.sub.c =E.sub.B +{E.sub.B.sup.2 +(I.sub.0 /C.omega.).sup.2 }.sup.-1/2.
The current I.sub.0 flowing through the primary coil 2 is indicated by I.sub.0 =(E.sub.B /L)t.sub.on, where t.sub.on is the period during which the transistor 4 is ON. That is, I.sub.0 is proportional to the ON period of the transistor 4. Therefore, the peak or high value of the flyback pulse can be varied by controlling the ON period of the transistor 4, thereby enabling the high output voltage of the secondary coil 7 to be stabilized.
In this way, when the high output voltage is to be stabilized by controlling the ON period of the transistor 4 in the circuit of FIG. 1 as it is, a reverse current flows from the ground toward the primary coil 2 through the damper diode 5 during the damper period (ON period of the damper diode 5). When this flow of the reverse current makes the voltage of the point A return to the power source voltage and the damper diode 5 is rendered nonconductive, the current supplied from the driving power source 3 toward the primary coil 2 begins to flow through the resonance capacitor 6 toward the ground because the transistor 4 has already been turned OFF. This causes series resonance to occur in the primary coil 2 and the resonance capacitor 6, whereby, as shown in FIG. 2(a), a useless pulse P.sub.W is generated in the period from the end of the damper period to the subsequent ON state of the transistor 4.
The pulse P.sub.W causes a noise to be generated so as to adversely affect the circuit operation. Accordingly, the circuit of this type is so organized that a current flow from the primary coil 2 to the resonance capacitor 6 is prevented from occurring in the end portion of the damper period. More specifically, a switching device for interrupting the current is connected in the path from the driving power source 3 to the ground through the primary coil 2 and the resonance capacitor 6 or the transistor 4. Alternatively, the starting portion of the ON period of the transistor 4 is set to overlap with the end portion of the damper period (FIG. 2(d)) as indicated by a broken line in FIG. 2(c), so that the current from the side of the driving power source 3 flows toward the transistor 4.
Since the ON period of the transistor 4 overlaps with the damper period, however, the transistor 4 cannot control the output on the basis of the timing of the ON operation. In a circuit which provides a switching device for interrupting the current in the path from the driving power source 3 to the ground through the primary coil 2 and the resonance capacitor 6 or the transistor 4, the switching device must be provided separately from the transistor 4 in order to block the current, and a control circuit for controlling the operation timing of the switching device is further required. This increases the number of parts and makes the circuit configuration complex, thereby producing a problem in that the cost of such a circuit is high.